There is a small sect of car enthusiasts known as hypermilers who go to extreme measures to get the most MPGs out of their cars as they can. Usually use a light and efficient car to begin with, but not John Watkinson of the U.K. He modified his 1984 Jaguar XJS to achieve the equivalent of 43 MPG, detailing the litany of changes in a story for The Register.

I’ve always said the greenest car is the one that’s already on the road, and Watkinson is proof that a little elbow grease and a change to your driving habits can make a huge difference. So even though his car used a 3.6 liter inline-six instead of the optional V12 for motivation, gas mileage still averaged in the mid-to-high teens for most owners.

Watkinson got to work on his Jag by removing about 500-lbs of excess weight, including replacing 57-lbs of copper wire with aluminum and replacing the electric window mechanism with a lighter unit from Audi. These and other modifications brought the 3,679-lb Jaguar down to under 3,300-lbs, which makes getting going a whole lot easier. While the XJS was surprisingly aerodynamic for an 80s car, he did add an underbody tray to smooth out the airflow underneath.

More impressive to this story though is that through a combination of engine efficiency improvements, Watkinson made the old I6 a veritable fuel-sipper. An electric water pump and a new radiator with active grille shutters improve cooling and reduce parasitic power loss on the engine. He did the same thing with the power steering system, and he further modified the air conditioner and alternator to only draw power when the car was decelerating. Those modifications might fall on the more “extreme” side of hypermiling if you live in a hot and humid climate, but for more mild climates it’s not such a big deal. He also changed the final drive ratio from an aggressive 3.54:1 to a more fuel-friendly 2.88:1, which means slower acceleration but better efficiency.

All told, Watkinson says these modifications enabled him to make a 630 mile trip on a single tank of petrol, and he claims that his car corresponds to a 43 MPG vehicle on the Imperial standard. In America that translates to about 36 MPG, though if we were to assume that his 630 mile trip left him running on fumes, it’s only about 31.6 MPG. That’s still much better than stock, and demonstrates that old cars can be made a lot more efficient through some clever and targeted modifications.

While you could run out and buy the new 75 MPG Jaguar XE, if you’re the DIY type who wants to keep some classic cars on the road a little bit longer, going the hypermiling route might not be a bad alternative.

Do you think we could make a classic American muscle car get 30 MPG or more? I kinda want to find out.